A combined analysis of hydrochemical, isotopic, microbiological and geophysical characteristics for assessment of nitrogen loading to a deep crystalline bedrock aquifer

A combined analysis was conducted to understand nitrogen loading to a deep (~ 80 m below ground level (bgl)) bedrock aquifer, including hydrochemical, isotopic, microbiological, hydrogeological, and geophysical investigation. The studied site, located in a suburban area, had been used to grow fodder crops for cattle and fertilized with cattle manure. The geology consists of Gyeonggi gneiss complex, fine-grained granite, and alluvium, while the surface is covered by loess deposits. Four boreholes (BH-1~4) were installed in 2018-2019, and geological, hydrogeological and geophysical surveys were conducted. Then hydrochemical, isotopic and microbiological properties were quarterly studied for 4 years at both shallow (~ 30 m bgl) and deep (~ 80m bgl) groundwater in each borehole as well as a pre-existing well. Groundwater levels were automatically monitored using levelogger. The initial depth to groundwater was 20.4 – 24 m bgl. As a result, high concentrations of nitrate were consistently observed in deep groundwater (35.3±16.0 mg/L; n=59) as well as shallow groundwater (50.9±16.8 mg/L; n=60) and a pre-existing well (44.3±6.8 mg/L; n=16). Based on N isotopes of nitrate (8.9-19.0‰; n=27), nitrogen mainly came from manure or sewer. The enrichment of ¹⁵N along with decreasing dissolved oxygen and nitrate in deep groundwater indicated denitrification in deep subsurface. Meanwhile ammonium and nitrite were exceptionally high in two deep groundwaters (BH-1d; BH-4d) where fecal coliforms were also occasionally detected. δ¹⁵N of ammonium were 9.2 and 14.0‰ in BH-1d and BH-4d, respectively. The hydrochemical, isotopic and microbiological results indicate that the vertical transport of contaminants from surface to deep groundwater is significant in the study area, probably through permeable fractures, given that the integrated interpretation of seismic data and electricity resistivity with geological data suggested fractures in deep weathered soil zones (down to 48 m bgl) and soft rocks. In addition, the borehole logging identified permeable fractures in hard rocks at high dips particularly in BH-1 and BH-4 in the west (up to 72. 9 degree). The natural gamma ray and P-wave velocity were higher in the west, indicating the different geology from the east (BH-2 and BH-3). Besides, the groundwater levels were fluctuated at BH-1d and BH-4d, due to groundwater extraction nearby. This combined examination of hydrochemical, isotopic, microbiological, hydrological, and geophysical characteristics suggests a scenario of contaminant transport from surface to deep subsurface through permeable fractures, which is enhanced by groundwater pumping. The integrated analysis is expected to be useful for subsurface characterization in crystalline bedrocks. <Acknowledgement> This study was supported by the basic research project of Korea Institute of Geoscience and Mineral resources (KIGAM) funded by the Ministry of Science and ICT of Korea (No. 23–3411) and by the Korea Environment Industry & Technology Institute (KEITI) through the Subsurface Environment Management Research Project (No. 2021002440003).